Method of grease manufacture



METHOD OF GREASE MANUFACTURE Filed May 22, 1963 NW NW United States Patent 3,242,087 METHGD 0F GREASE MANUFACTURE Clarence L. Dowdeu, .lr., and William R. Coons, .lr., Port Arthur, and Herbert J. Pitman, Groves, Tex., assignors to Texaco Inc., New York, N.Y., a corporation of Delaware Filed May 22, 1963, Ser. No. 282,340 8 Claims. (CL 25241) This invention relates to improvements in the manu facture of lubricating greases. More particularly, it relates to an improved method for the manufacture of greases thickened with sodium fatty acid soaps.

The preparation of sodium soap greases by the conventional procedure requires very long manufacturing times, involving particularly very long cooling times with the slow addition of a large amount of cold oil to the concentrated soap slurry obtained in the saponification step, which is necessitated by the strong tendency of the concentrated soap slurry to lump formation during this oil addition. The use of suflicient amounts of lubricating oil in the saponification mixture to substantially reduce this difliculty is impractical because of the much longer saponification and dehydration periods required with the more dilute saponification mixtures, and particularly because of the resultant greatly increased foaming difficulties during the dehydration step.

The present invention provides a convenient means of overcoming the difficulties of the prior art methods, and of producing sodium soap greases in greatly reduced manufacturing times, amounting to savings of about 10 hours or more for a 40,000 pound batch, without sacrifice in yields or product quality. The method of our invention comprises the step of subjecting the grease mixture at an elevated temperature to shearing of a particular type, which is very suitably obtained by recirculating the grease mixture through a shear valve with a pressure drop across the valve from about 40 to about 200 pounds per square inch. We have found that shearing in this manner at a sutliciently elevated temperature very effectively disperses any lumps and also increases the grease yield without undesirably affecting the grease texture, very differently from the effect of shearing by essentially different meth ods or of shearing at lower temperatures to the extent necessary to disperse lumps. This method is adapted particularly for the production of sodium soap thickened greases of a slightly stringy texture which is required for a good automotive wheel bearing grease.

The temperature at which the shearing is carried out is in the range from above about 300 F. down to a low temperature thermal transistiorr range, occurring at about l90230 F. for a sodium tallowate-mineral oil grease. It is generally preferable to discontinue the shearing at a temperature substantially above this range, such as above about 250 F., and preferably not below about 270 F. in the preparation of a sodium tallowatemineral oil grease. The shearing is preferably carried out while the grease mixture is cooled from the top temperature above 300 F. down to at least about 280 F. In some cases, it is desirable to continue shearing the grease mixture down to and within the low temperature transistion range with relatively mild shearing obtained by employing a pressure drop below about 30 pounds per square inch across the shear valve.

The preferred embodiment of this invention comprises continuously recirculating the grease mixture through a recycle line on the grease making zone during the cooling cycle with shearing of the recirculated stream of grease mixture during the first part of the cooling cycle as described above and with introduction 3,242,087 Cfi Patented Mar. 22, 1966 of the additional oil added during cooling into the recirculating stream of grease mixture, as described in copending application Serial No. 282,330 of L. F. Badgett, W. R. Hencke and F. T. Crookshank, filed of even date herewith.

A particularly preferred embodiment of this invention comprises carrying out saponification and dehyra tion with a reduced amount of lubricating oil in the kettle charge, and adding preheated oil as the kettle contents are heated to top temperature. The preheated oil may be very advantageously added to a recirculating stream of grease mixture, as also described in the said application of Badgett et al. However, when the grease mixture is too stilt to circulate, the preheated oil may be added through the top of the kettle.

The figure is a diagrammatic illustration of one form of apparatus suitable for making greases in accordance with the preferred embodiment of our invention.

Referring in detail to the figure, numeral 1 represents a jacketed grease kettle equipped with a stirrer 2 and adapted to be heated to temperatures up to at least about 350 F. In carrying out the grease preparation, the grease kettle is charged with saponifiable material, lubricating oil and sodium hydroxide in approximately the stoichiometric amount required to react With the saponifiable material. If desired, a small excess of sodium hydroxide may be employed, such as to give a grease containing up to about 0.5 percent by weight of free alkali. The sodium hydroxide is employed in the form of a water solution, preferably comprising only sufiicient water to completely dissolve the sodium hydroxide. These materials may be introduced into the kettle in any desired order. The oil and saponifiable material may be employed in a weight ratio from about 0.25:1 up to about 5:1, respectively, most suitably in about a ratio from 0.511 to about 4:1.

The kettle contents are heated with stirring until saponification and dehydration are substantially complete, and then heated further to the top temperature in a period of time from about 45 minutes to about 3 hours, including a holding period in about the range 210220 F. of at least about 15 minutes, and preferably about one half hour. The maximum temperature to which the grease mixture is heated is in the range from about 300 F. to just below the melting point of the soap. It is preferably a temperature in the range from about 300 F. to about 330 F. for a sodium tallowate grease.

Recirculation of the grease mixture from the bottom of kettle 1 to a point above the level of the material in the kettle is carried out during the cooling, and very advantageously during at least a portion of the heating cycle also when the grease mixture is sulficiently fluid to circulate. The recirculation is begun by turning valves 6 and 8 to the open position and starting pump 12. The grease mixture passes through line 5, containing valve 6, line 10, containing valve 8, pump 12, and line 14 containing pressure gauge 15 and shear valve 10, which is suitably a gate valve. Valve 19 is located a sufficient distance from kettle 1 to avoid spraying of the grease mixture into the kettle when the valve is operated under a back pressure, suitably a distance equal to at least about 12 times the diameter of pipe 14. Lines 5, 10 and 14 may be jacketed or otherwise provided with means for applying additional heating and cooling to the recirculating stream of grease mixture.

Circulation of the grease mixture through the recycle system during the heating is carried out at a rate sufiicient to give one batch turnover within about 22 minutes, such as in about 0.3-22 minutes, and preferably in about 0.4-15 minutes, based on the Weight of the grease mixture during the heating cycle, or in about 0.25 minutes, and preferably about 0.312 minutes, based upon the average weight of grease mixture during the heating cycle when the process is carried out with additional oil added during the heating cycle as described hereinbelow. Recycling during the cooling cycle is suitably carried out at a rate sufficient to provide a batch turnover within about 35 minutes, and preferably within about 20 minutes, based on the weight of the finished grease, or within about 27 minutes, and preferably within about 17 minutes, based on the average weight of grease mixture during the cooling cycle. Recycling during the cooling from the top temperature down to about 270 F. is preferably carried out at a rate providing a batch turnover within about minutes, and most suitably in about 0.2510 minutes, based on the average weight of grease mixture within the kettle during this cooling period.

Recirculation of the grease mixture is preferably carried out continuously during the cooling from above about 300 F. down to about 270 R, which may require from about 15 minutes to about 2 hours, at a rate providing at least about 5 batch turnovers. Under our preferred conditions, the grease mixture is cooled in this range in about 30 minutes to about one hour with recirculation at a rate giving at least about 10 batch turnovers, based on the average weight of the grease mixture during this period.

Cooling of the grease mixture is carried out by cutting off the heat to the kettle, and to the recycle line if such additional heating is employed, and introducing lubricating oil from tank 20 into the recirculating stream of grease mixture. The added oil is at a temperature substantially below that of the grease mixture, such as at least 100 F. below the temperature of the grease mixture at the beginning of the cooling step. The lubricating oil passes from tank 20 into line 36 by Way of line 22 containing valve 23, pump 24, line 25 and line 26 containing valve 27, dial thermometer 33, pressure gauge 34 and valve 35. Valve 35 is preferably a one way valve, most suitably of a type designed to prevent gravity flow of oil through pump 24 when the pump is not operating, such as a diaphragm controlled reducing valve or a spring loaded check valve. In line 36, the oil may pass into the recirculating stream of grease mixture at the intake of pump 12 by passing through valve 37 into line 10, or into the grease mixture at the discharge side of pump 12 by passing through valve 38 into line 14. It is preferably passed into the grease mixture at the intake side of pump 12 in order to obtain increased mixing by the action of the pump. The rate of oil addition during the cooling from the top temperature down to about 270 F. is suit ably such that the ratio of the rate of flow of the grease mixture before the point of confluence to the rate of oil injection is at least about 10:1, and may be as high as about 500:1, respectively. It is preferably at a rate such that the ratio of the rate of flow of grease mixture to the rate of oil injection is from about 50:1 to about 300:1, respectively. Below about 270 F., the rate of addition of cold oil is relatively noncritical, and the oil may be introduced at a faster rate, suitably at a rate giving a ratio of grease recirculation rate to oil injection rate from about 2:1 to about 150:1, respectively. Additional cooling may be applied to the kettle and also to the recirculating stream of grease mixture. The cooled grease mixture is finally drawn through line 40, containing valve 41.

During the cooling from the top temperature down to about 270 F., the recirculation is carried out with shear valve 19 set in a partly closed position so as to give a pressure drop of about 40200 pounds per square inch, and preferably about -125 pounds per square inch across the valve. Shearing may be continued during further cooling of the grease mixture down to the drawing temperature if desired with a pressure drop below 30 pounds per square inch, such as about 5-25 pounds per 4- square inch pressure drop across the valve. Such shearing is advantageously employed in SOrne cases when the recirculation of the grease mixture is carried out at a rate resulting in below about 10 batch turnovers in the high temperature range.

In grease preparations by the method comprising a particularly preferred embodiment of this invention, the saponification is carried out in the presence of lubricating oil in a weight ratio below about 1.25:1, and most advantageously, below about 1:1, respectively, with the saponifiable material, and additional oil required to give a ratio of oil-soap from about 1.25:1 to about 5:1, and preferably from 1.25:1 to 2: 1, respectively, is added during the heating to top temperature, following substantial dehydration of the grease mixture. The oil is preferably added at a temperature substantially higher than that of the grease mixture, very advantageously at a temperature above 300 F. up to the melting point of the soap. It is preferably preheated to a temperature in the range from about 310 F. to about 360 F. When the grease mixture is sutficiently fluid to circulate, the preheated oil is preferably introduced into the recirculating stream of grease mixture in the same manner as the oil added during cooling as described above. The oil passes from tank 20 to heater 30 by way of line 22, pump 24, line 25 and line 28 containing valve 29. Heater 30 may be any suitable type of heater, such as a coil heater, as indicated in the diagram. From heater 30 the oil passes into line 26 and is introduced into the circulating stream of grease mixture at either the intake or the discharge side of pump 12 as described hereinabove in connection with the intro duction of oil during the cooling. It is preferably added at the intake of pump 12 except in the case of very light oils which may cause vapor locking in the pump. The rate of oil addition is preferably such that the ratio of the rate of flow of grease mixture before the point of confluence to the rate of oil injection is at least about 10:1, and may be as high as about 500:1, respectively. It is very suitably at a rate such that the ratio of the rate of flow of grease mixture to the rate of oil injection is from about 20:1 to about 200: 1, respectively.

The oil addition is preferably carried out in the above manner with shearing of the grease mixture by means of valve 19 set in a partly closed position to give a pressure drop of from about 10 to pounds per square inch. Shearing by means of a pressure drop in the range from about 10 to about 35 pounds per square inch is generally sufiicient to disperse any lumps formed during this oil addition. When the oil is added at the kettle top, recirculation with shearing is preferably begun as soon as the kettle contents become sufficiently fluid to circulate, and carried out so as to give at least two complete batch turnovers before the beginning of the cooling cycle.

When the saponification is carried out upon a mixture comprising lubricating oil in an amount giving a weight ratio with the saponifiable material above about 1:1, respectively, recycling is preferably carried out during the dehydration step with the grease mixture returned to the kettle at a point above the surface of the reaction mixture as indicated in the diagram, as a means of reducing or overcoming the foaming difficulty with these more dilute soap mixtures. The recycling is carried out with the shear valve in the wide open position during the dehydration step. Very advantageously, when employing a relatively dilute saponification mixture, the recirculation may be started immediately upon charging the kettle and continued throughout the process, with the introduction of preheated oil into the recycle stream during heating and of oil at a lower temperature into the recycle stream during cooling as described hereinabove.

The sodium fatty acid soap greases to which this invention relates comprise an oleaginous liquid lubricating base as the chief constituent and a grease thickening amount of sodium fatty acid soap. They ordinarily contain the sodium fatty acid soap in an amount from about 3 to about 30 percent by weight, although somewhat higher or lower amounts may also be employed. The soap is obtained by the saponification of a higher fatty acid containing 12 to 32 carbon atoms per molecule, the glyecrides or other esters of such acids and mixtures thereof. A particularly suitable saponifiable material is a tallow containing about 1-10 percent of free fatty acids and having an iodine number from about 40 to 80.

The lubricating oils forming the major constituent of these greases may be any oils of lubricating characteristics which are suitable for use in lubricating greases generally. Such oils include particularly the conventional mineral lubricating oils having Saybolt Universal viscosities in the range from about 75 seconds at 100 F. to about 225 seconds at 212 P. which may be either naphthenic or paraffinic oils or blends thereof. The preferred mineral oils are those having Saybolt Universal viscosities in the range from about 300 seconds at 100 F. to about 100 seconds at 210 R, which may be blends of lighter and heavier oils in the lubricating oil viscosity range.

Synthetic lubricating oils which may be preferred for obtaining greases having special properties required for certain types of lubricating service include oils prepared by cracking and polymerizing products of the Fisher Tropsch process and the like, as well as other synthetic oleaginous compounds such as diesters, polyesters, polyethers, etc., having viscosities within the lubricating oil viscosity range. Examples of suitable diesters include the olefinic dicarboxylic acid diesters, such as di-2-ethylhexyl sebacate, di(secondary amyl) sebacate, di-2-ethylhexyl azelate, di-iso-octyl adipate, etc. However, a lubricating oil which is substantially unreactive under the saponification conditions is preferably employed in the saponification mixture. Lubricating oils employed for this purpose are suitably mineral oils having viscosities of at least 300 seconds Saybolt Universal at 100 R, which may be mixtures of lighter and heavier oils.

Various additives of the usual types such as corrosion inhibitors, oxidation inhibitors, extreme pressure agents, etc. may be employed in these greases. Suitable oxidation inhibitors include particularly those of the amine type, such as, for example, diphenylarnine, phenylalphanaphthylamine and tetramethyl diamino diphenyl methane. Such compounds may be added to the grease mixture during cooling, preferably when the grease mixture is below about 250 F.

The following example is given for the purpose of further disclosing the invention.

Example I A lubricating grease comprising a mineral lubricating oil thickened with sodium tallowate was prepared by the method of this invention employing shearing of the grease mixture at high temperatures in the manner described below.

The following materials were employed in this preparation: The mineral oil employed was a propane deasphalted and dewaxed residuum from a paraffin base crude having a Saybolt Universal viscosity of 192 seconds at 210 F. and a pour point of about 44 F. The saponifiable material employed was a commercial tallow having a saponification number of 198, an iodine number of 51, a titer of 41.0 C. and a free fatty acid content of 4.9 percent.

The equipment employed was a 150 pound capacity steam heated laboratory kettle having auxiliary equipment for grease recirculation with shearing and with hot and cold oil injection into the recycle stream as shown in FIG. 1. The circulation equipment consisted of a 1% inch pipe connecting the kettle drawoif with a No. 2 Globe Rota Piston pump and a inch pipe extending from the pump to the top of the kettle, containing a gate valve employed as a shear valve.

Following is a detailed description of the method employed in the grease preparation: The kettle was charged with 9.2 pounds of the mineral oil, 18.3 pounds of the tallow and 5.51 pounds of 49 percent caustic solution. Heating of the kettle contents was begun with stirring and with recirculation through the recycle line at a rate of 17 gallons per minute with the shear valve in the wide open position. The temperature of the mixture was brought up to 210 F. in about 13 minutes, and then maintained in the range from 210 to 225 F. for 30 minutes. The mixture was further heated to 290 F. in 40 minutes and the injection of additional oil preheated to about 340 F. into the recirculating stream of grease mixture was begun at a rate of 1.31 pounds per minute. Injection of the oil was continued during the heating to 312 F. in 14 minutes until 18.3 pounds of oil had been introduced. The mixture was then maintained at a temperature from 312 F. to 314 F. for an additional 30 minutes. After this holding period the heat was cut off and the injection of 25 pounds of oil at 160 F. was begun at a rate of 1.09 pounds per minute. Shearing of the recirculating stream of grease mixture was also begun at the end of the holding period with the shear valve set to give a pressure drop of 60 pounds per square inch and continued during the cooling from the top temperature of 314 F. down to 269 R, which required 23 minutes. Thereafter 35.7 pounds of the mixture was removed and 99.3 pounds of additional oil at a temperature of F. were injected at a rate of 1.95 pounds per minute with the shear valve in the wide open position. The total manufacturing time was about 3 hours and 11 minutes.

The product obtained as described above was a grease of excellent texture and appearance and obtained in a good yield. The following analytical and test results were obtained upon this grease.

Composition, percent:

Sodium soap 6.3 Free alkali None Free fatty acid 0.07 Free neutral fat 0.01 Water Trace Mineral oil Remainder ASTM penetration at 77 F Unworked 351 Worked, 60 strokes 362 Worked, 100,000 strokes. 384 Dropping point, F., ASTM 304 The above grease was equivalent in all respects including shear stability, oxidation resistance, water resistance, etc. to the products obtained by regular plant manufacture.

Example II A different sodium tallowate grease was prepared by the method of our invention as described below.

The following materials were employed in this preparation: The lubricating oil was a refined residuum from a paraflin base crude having a Saybolt Universal viscosity at 210 F. of 177.5 seconds and a pour point of 105 F. The saponifiable material employed was a commercial hard tallow having a saponification number of 198, an iodine number of 50 and a free fatty acid content of 4.4 percent.

The equipment employed was a jacketed steam heated pound laboratory kettle having auxiliary equipment for grease recirculation with shearing and oil injection as described in Example I.

Following is a detailed description of the method employed in the grease preparation: The kettle was charged with 9.6 pounds of mineral oil, 19.2 pounds of tallow and 5.71 pounds of 49 percent sodium hydroxide solution. Heating and stirring of the kettle contents were begun and the kettle contents brought up to 210 F. in about 10 minutes. The temperature of the mixture was maintained at 210212 F. for 30 minutes, and then brought up to 270 F. in an additional 17 minutes. The addition of 19.2 pounds of the mineral oil preheated to 348 F. was then begun at a rate of 0.64 pound per minute through the kettle top. When 8.3 pounds had been injected, circulation of the grease mixture through the recycle line was begun at a rate of 2.8 gallons per minute with the shear valve set to give a pressure drop of 30 pounds per square inch across the valve. The grease mixture was heated from 270 F. to 315 F. in 19 minutes during the oil addition. The kettle heating was then discontinued and 71.4 pounds of oil at about 150 F. were introduced into the recycle stream of grease mixture at a rate of 0.64 pound per minute with the shear valve set to give a pressure drop of 60 pounds per square inch during the cooling down to 270 F. The shear valve was then set to give a pressure drop of 30 pounds per square inch across the valve during the remainder of the cooling with oil addition. The cooling time was one hour and 49 minutes, the total manufacturing time being 3 hours and 35 minutes.

A firm grease of the desired stringy texture was obtained as described above, having an ASTM penetration of 152 unworked, 219 worked, at a 16.5 percent soap content.

A grease preparation carried out in substantially the same manner with circulation during the oil addition in the cooling step but without shearing had an ASTM penetration of 189 unworked, and 262 worked, at a 16.5 percent soap content, and was very lumpy.

Obviously, many modifications and variations of the invention, as hereinbcfore set forth,,may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. The process of preparing a lubricating grease comprising as the major component a lubricating oil thickened to a grease consistency by a sodium fatty acid soap which comprises providing a saponification mixture consisting essentially of a soap forming fatty acid material, sodium hydroxide in an amount corresponding approximately to the stoichiometric amount required to react with the said soap forming material, and a lubricating oil which is substantially non-reactive under the saponification conditions in an amount equal to from about 0.25 to about 5 times the weight of the said saponifiable material, heating the said mixture up to a maximum temperature in the range from about 300 to about 330 F. and thereafter cooling the grease mixture with the addition of lubricating oil at a substantially lower temperature than the grease mixture, and subjecting the grease mixture to shearing for at least 15 minutes at a temperature in the range from about 300 F. on the heating cycle to about 270 F. on the cooling cycle by continuously withdrawing a minor stream of grease mixture from a maintained body of said mixture, passing it through a shear valve with a pressure drop of about 40200 pounds per square inch across the said valve and returning the said stream to the maintained body of the said mixture at a rate such that the amount of circulated grease mixture equals the total average amount of grease mixture during this period within about 15 minutes and at least 5 batch turnovers are obtained, said grease mixture being diluted during the heating with any additional oil required to give an oil-soap ratio of 1.25:1 to 5:1, respectively, said dilution being carried out without any substantial cooling of the grease mixture.

2. The process of claim 1 wherein the said saponifiable material is selected from the group consisting of C -C fatty acids and their giycerides.

3. The process of claim 1 wherein the said saponifiable material is tallow, having an iodine number in the range 40-80 and comprising 1-10 percent of free fatty acids.

4. The process of claim 1 wherein the said lubricating oil is a mineral lubricating oil.

5. The process of claim 1 wherein the recycling during cooling from above about 300 F. to about 270 F. is carried out so as to provide at least about 10 batch turnovers.

6. The process of claim 1 wherein the initial charge comprises lubricating oil and saponifiable material in a weight ratio from 0.25:1 to 1:1, respectively and additional oil at a temperature above about 300 F. is introduced during the heating cycle to give an oil-soap ratio in the range from 1.5:1 to 2:1, respectively.

7. The process of claim 1 wherein oil at a substantially lower temperature is introduced into the grease mixture during the cooling from the maximum temperature down to about 270 F. at a rate such that the ratio of the rate of recirculation of grease mixture to the rate of oil injecttion is in the range from about 10:1 to about 500:1, respectively.

8. The process of claim 7 wherein the said ratio is in the range from about 50:1 to about 300:1, respectively.

References Cited by the Examiner UNITED STATES PATENTS 2,332,202 10/1943 Calkins 252--42.1 2,450,220 9/1948 Ashburn et al. 252-41 2,830,022 4/1958 Nelson et al. 25241 2,870,090 1/1959 Pitrnan et al. 25289 2,886,525 5/1959 Dilworth et al. 252-39 3,015,624 1/1962 Hencke et al. 25241 3,068,174 12/1962 Pelton et al. 25239 3,068,175 12/1962 Roach et al 252-39 3,079,341 2/1963 Coons et al 25241 3,117,087 1/1964 McCormick et al 2524l DANIEL E. WYMAN, Primary Examiner.

I. VAUGHN Assistant Examiner. 

1. THE PROCESS OF PREPARING A LUBRICATING GREASE COMPRISING AS THE MAJOR COMPONENT A LUBRICATING OIL THICKENED TO A GREASE CONSISTENCY BY A SODIUM FATTY ACID SOAP WHICH COMPRISES PROVIDING A SAPONIFICATION MIXTURE CONSISTING ESSENTIALLY OF A SOAP FORMING FATTY ACID MATERIAL, SODIUM HYDROXIDE IN AN AMOUNT CORRESPONDING APPROXIMATELY TO THE STOICHIOMETRIC AMOUNT REQUIRED TO RACT WITH THE SAID SOAP FORMING MATERIAL, AND A LUBRICATING OIL WHICH IS SUBSTANTIALLY NON-REACTIVE UNDER THE SAPONIFICATION CONDITIONS IN AN AMOUNT EQUAL TO FROM ABOUT 0.25 TO ABOUT 5 TIMES THE WEIGHT OF THE SAID SAPONIFIABLE MATERIAL, HEATING THE SAID MIXTURE UP TO A MAXIMUM TEMPERATURE IN THE RANGE FROM ABOUT 300 TO ABOUT 330*F. AND THEREAFTER COOLING THE GREASE MIXTURE WITH THE ADDITION OF LUBRICATING OIL AT A SUBSTANTIALLY LOWER TEMPERATURE THAN THE GREASE MIXTURE, AND SUBJECTING THE GREASE MIXTURE TO SHEARING FOR AT LEAST 15 MINUTES AT A TEMPERATURE IN THE RANGE FROM ABOUT 300*F. ON THE HEATING CYCLE TO ABOUT 270*F. ON THE COOLING CYCLE BY CONTINUOUSLY WITHDRAWING A MINOR STREAM OF GREASE MIXTURE FROM A MAINTAINED BODY OF SAID MIXTURE, PASSING IT THROUGH A SHEAR VALVE WITH A PRESSURE DROP OF ABOUT 40-200 POUNDS PER SQUARE INCH ACROSS THE SAID VALVE AND RETURNING THE SAID STREAM TO THE MAINTAINED BODY OF THE SAID MIXTURE AT A RATE SUCH THAT THE AMOUNT CIRCULATED GRASE MIXTURE EQUALS THE TOTAL AVERAGE AMOUNT OF GRASE MIXTURE DURING THIS PERIOD WITHIN ABOUT 15 MINUTES AND AT LEAST 5 BATCH TURNOVERS ARE OBTAINED SAID GREASE MIXTURE BEING DILUTED DURING THE HEATING WITH ANY ADDITIONAL OIL REQUIRED TO GIVE AND OIL-SOAP RATION OF 1.25:1 TO 5:1, RESPECTIVELY SAID DILUTION BEING CARRIED OUT WITHOUT ANY SUBSTANTIAL COOLING OF THE GREASE MIXTURE. 